1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device which can share a memory with another device.
The present invention also concerns an electronic system having a plurality of functional circuits which are operated while being switched in terms of time and each of which processes data while accessing the memory.
2. Description of the Background Art
Referring to FIG. 10, an explanation will be given of the outline of a conventional DVD recorder system (DVD: Digital Versatile Disc or Digital Video Disc). Upon recording moving pictures, an encoder 1 carries out an encoding process in which inputted video data is compressed, for example, in conformity with the MPEG2 Moving Picture Experts Group phase 2) standard. The resulting compressed data is recorded in a DVD 2 that serves as a recording medium. Upon reproducing the moving pictures, a decoder 3 carries out a decoding process in which the compressed data, inputted from the DVD 2, is decompressed in conformity with the MPEG2 standard. Thus, the moving pictures are reproduced on a display based upon the video data outputted from the decoder 3.
Dedicated memories 4 and 5 are respectively connected to the encoder 1 and the decoder 3. In order to process video data on the MPEG2 standard, a memory of not less than 64 M bits is required at the time of encoding, and that of not less than 32 M bits is required at the time of decoding; that is, a large-capacity memory is required. Here, a memory 4 is constituted by four SDRAMs (Synchronous Dynamic Random Access Memory), each having, for example, 16 M bits, and a memory 5 is constituted by two SDRAMs, each having 16 M bits.
In this arrangement, upon recording and upon reproducing, only either of the corresponding encoder 1 and decoder 3 is operated and only the memory connected to the operated side is accessed. Consequently, even though only four of the SDRAMs are operated at one time in the maximum, six SDRAMs are used. Such an inefficient application of the memories causes an increase in the system costs.
Therefore, one of objectives of the present invention is to provide a semiconductor integrated circuit device which can share a memory with an external device.
Another objective of the present invention is to provide an electronic system in which a plurality of functional circuits that respectively carry out predetermined data processing are allowed to access a shared memory.
In one preferred mode of semiconductor circuit device of the present invention, a first selector, which selectively outputs a first access signal outputted from a functional block for executing a predetermined data processing and a second access signal received by an external interface terminal, is installed. The access signal selected by the first selector is given to a memory so as to access the memory.
A first external device for generating the second access signal is connected to the external interface terminal. The semiconductor integrated circuit and the first external device can access the shared memory so that it is possible to efficiently use the memory.
The semiconductor integrated circuit is further provided with a second selector for selectively making a judgment as to whether the functional block is connected to the external interface terminal or the first selector is connected to the external interface terminal. In the case when the semiconductor integrated circuit is allowed to use the memory, a signal can be transferred between the functional block and the external interface terminal, while in the case when another device is allowed to use the memory, one portion or all the portions of the second access signal is given to the memory from the external interface terminal through the two selectors. Therefore, the external interface terminal is sharedly used as the terminal for transferring a signal between the semiconductor integrated circuit and its external device and also as the terminal for receiving one portion or all the portions of the second access signal; thus, it is possible to reduce the number of terminals.
Here, it is preferable to construct the semiconductor integrated circuit by a single semiconductor chip having a built-in memory. Thus, the number of semiconductor chips through which the second access signal is transferred from the first external device to the memory is reduced; therefore, it is possible to shorten the accessing time to the first external device.
Moreover, in the case when an attempt is made so as to access another memory, this semiconductor integrated circuit is further provided with a third selector that selectively outputs a third access signal received by another external interface terminal and a fourth access signal generated in a functional block.
The second external device is connected to this external interface terminal, and the third access signal is given by the second external device. Therefore, the semiconductor integrated circuit and the second external device are allowed to access the common memory.
Here, the first and the second external devices may be provided as the same device (for example, LSI). Moreover, the semiconductor integrated circuit may be constituted by a single semiconductor chip in which the two memories are built.
More specifically, the functional block in the semiconductor integrated circuit executes an encoding process of data, and a device for executing a decoding process of data is connected to the external interface terminal. More preferably, the functional block is a circuit for carrying out a compressing process of video image data based upon MPEG, and the device connected to the external interface terminal carries out a decompressing process of the image data based upon MPEG.
Another preferred mode of the electronic system of the present invention is provided with a memory for storing data, a first functional block, formed on a single semiconductor chip, for carrying out a first data processing and for generating a first access signal used for accessing the memory; a second functional block, formed on another semiconductor chip, for carrying out a second data processing and for generating a second access signal used for accessing the memory; and a first selector for selectively applying to the memory the first access signal outputted from the first functional block and the second access signal outputted from the second functional block. Since the first and second functional blocks are allowed to access the common memory, the memory can be used efficiently.
Here, in the case when the first selector is formed on the same semiconductor chip as the first functional block, a second selector, which selectively makes a judgment as to whether a certain signal line is connected to the functional block or the signal line is electrically connected to the first selector, is preferably installed on the same semiconductor chip. The signal line is connected to the second and third functional blocks. The third functional block, which is formed on a semiconductor chip that is different from the first and second functional blocks, carries out a third processing while the first functional block is carrying out the first data processing, and transfers data between it and the first functional block.
The second selector transfers a signal between the first and the third functional blocks when the first functional block is allowed to use the memory, and supplies one portion or all the portions of the second access signal to the first selector when the second functional block is allowed to use the memory. Here, a commonly-used terminal that serves as the terminal for transferring data between the first and second functional blocks and that also serves as the terminal for receiving the second access signal from the second functional block can be formed on the semiconductor chip; therefore, it is possible to reduce the number of the terminals.
Moreover, it is preferable to construct the first functional block and the memory on the same semiconductor chip. Since the number of the semiconductor chips through which the second access signal passes through before being supplied to the memory from the second functional block is reduced; therefore, it is possible to shorten the access time from the second functional block to the memory.
Furthermore, the first functional block is an encoder circuit for executing a data compressing process, and the second functional block is a decoder circuit for executing a data decompressing process. More preferably, the first functional block executes a compressing process of video image data based upon MPEG and the second functional block executes a decompressing process of video image data based upon MPEG.
Additionally, in the semiconductor integrated circuit and the electronic system of the present invention, each of the first and second access signals includes an address signal for specifying a storage position in the memory and a control signal for controlling the operation of the memory. In the case when the memory is an SDRAM, the access signal includes a chip select signal, a low address strove signal, a column address strove signal, a write enable signal, a bank address signal and an address signal.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.